Oxygen Index Analysis of Ethylene-Propylene Rubber (EPR) Cable Compounds

The limiting oxygen index (LOI) is the key laboratory parameter used to quantify the flame-retardant performance of ethylene-propylene rubber (EPR) cable compounds. This technical article reviews EPR insulation and sheathing materials, the sample types, test items, and procedures specified in ASTM D2863, ISO 4589-2, and GB/T 2406.2, together with typical LOI values, influencing factors, and applications in flame-retardant wire and cable design.

Technical Overview

Ethylene-propylene rubber (EPR) is a saturated, non-polar polyolefin elastomer produced either as a binary ethylene-propylene copolymer (EPM) or as a terpolymer containing a diene monomer (EPDM). Peroxide- or silane-crosslinked EPR compounds are used extensively for cable insulation and sheathing because they combine stable dielectric properties, a continuous conductor operating temperature of 90 degrees Celsius (with short-circuit tolerance up to 250 degrees Celsius), excellent ozone, weather and chemical resistance, good water-tree resistance, and superior low-temperature flexibility.

Despite these advantages, EPR is fundamentally a hydrocarbon polymer and burns readily in air. The limiting oxygen index of a gum or conventionally filled EPR vulcanizate is typically only 21 to 24 percent, essentially at the level of ambient air, which contains approximately 21 percent oxygen. Flame-retardant EPR cable compounds therefore incorporate high loadings of mineral flame retardants, most commonly aluminum trihydrate (ATH) at 150 to 250 phr, often synergized with magnesium hydroxide (MDH), zinc borate, red phosphorus, or halogen-antimony systems, which raise the oxygen index to 28 percent or above.

The oxygen index (limiting oxygen index, LOI) is defined as the minimum concentration of oxygen, expressed as volume percent in a flowing oxygen-nitrogen mixture, that will just support candle-like combustion of a vertically mounted specimen for at least 3 minutes or over a burning length of at least 50 mm after ignition from the top. Because the test is quantitative, reproducible, and inexpensive, LOI is a primary screening and quality-control parameter for compound development, incoming material inspection, and specification compliance in the cable industry.

• LOI below 21 percent: the material continues to burn in normal air and is classified as easily combustible.
• LOI of 21 to 24 percent: marginal self-extinguishing behavior, typical of non-flame-retarded EPR insulation.
• LOI of 25 to 28 percent: flame-retardant range, achieved with moderate mineral filler loadings.
• LOI of 29 to 35 percent: high flame retardance, typical of halogen-free flame-retardant EPR insulation and sheathing.
• LOI above 35 percent: excellent flame retardance, obtained with optimized ATH/MDH systems or halogen-antimony synergy.

Testing Samples

EPR Insulation Compounds

Crosslinkable EPR insulation grades for low- and medium-voltage power cables, typically rated 0.6/1 kV through 35 kV and higher in specialized designs. These compounds are based on EPM/EPDM polymer, silane-treated calcined clay for electrical reinforcement, antioxidants, co-agents, and organic peroxide curing agents. Specimens are either fully compression-molded and cured plaques or strips cut longitudinally from the insulation wall of production cable, and the non-flame-retarded grades generally measure an LOI of 21 to 24 percent.

EPR Sheathing and Jacketing Compounds

Black sheathing compounds based on EPM/EPDM containing carbon black and mineral fillers, and, for flame-retardant grades, heavy ATH or ATH/MDH loadings. Flame-retardant EPR sheathing compounds typically achieve LOI values of 28 to 32 percent, while halogen-free low-smoke formulations can reach 30 to 35 percent. Test specimens are molded slabs of approximately 3 mm thickness or strips cut from the cable sheath with trimmed, defect-free edges.

Flame-Retardant and Specialty EPR Compounds

Additional sample categories include halogen-free flame-retardant (LSZH) EPR, semi-conductive EPR screening compounds, peroxide-cured versus silane-crosslinked formulations, and radiation-resistant EPR for nuclear-class cable applications. For each category, the state of cure, filler system, and specimen origin (molded compound versus finished cable) must be documented because all three factors influence the reported oxygen index.

Specimen Forms and Dimensions

In accordance with ISO 4589-2 and ASTM D2863, specimens are prepared as rectangular strips, typically 80 to 150 mm long, 10 mm wide, and 3 to 4 mm thick. When specimens are taken from finished cable, the actual wall thickness and any flattening of curvature must be recorded, since the oxygen index of rubber compounds depends measurably on specimen thickness.

Testing Items

The core testing item is the ambient-temperature limiting oxygen index. Supplementary flammability-related items are frequently specified alongside LOI to fully characterize flame-retardant EPR cable compounds:

• Ambient-temperature limiting oxygen index at 23 degrees Celsius per ASTM D2863, ISO 4589-2, or GB/T 2406.2.
• Elevated-temperature oxygen index per ISO 4589-3 at 70 to 120 degrees Celsius, and derivation of the temperature index at which the LOI falls to 21 percent.
• LOI retention after thermal air-oven aging, for example 135 degrees Celsius for 168 hours, to assess long-term flame-retardance stability.
• Complementary compliance tests, including single cable vertical flame propagation (IEC 60332-1 / GB/T 18380.12), smoke density (IEC 61034), and halogen acid gas evolution (IEC 60754).
• Correlation data such as UL 94 horizontal and vertical burn ratings and cone calorimeter heat-release measurements.

Common specification thresholds require LOI of at least 28 percent for flame-retardant EPR cable compounds, with enhanced safety classes frequently specifying 30 percent or higher.

Testing Methods

Specimen Preparation and Conditioning

Compound pellets or milled slabs are compression-molded into plaques and cured to the full crosslink state recommended by the compound supplier. Specimens are die-cut with smooth, defect-free edges, and a reference mark is placed 50 mm below the top edge to judge the burning-length criterion. Prior to testing, specimens are conditioned at 23 plus or minus 2 degrees Celsius and 50 plus or minus 5 percent relative humidity for at least 40 hours, or as required by the applicable product standard.

Apparatus and Test Procedure

The standard oxygen index apparatus consists of a vertical heat-resistant glass chimney approximately 500 mm high with an internal diameter of at least 70 mm, a gas-dispersion base packed with glass beads, oxygen and nitrogen supplies with pressure regulators and calibrated flowmeters or mass-flow controllers, a vertical specimen holder that clamps the lower end of the specimen on the chimney axis, and a small propane or butane igniter producing a 10 to 15 mm flame. The total mixed gas flow is controlled to give a linear velocity of 40 plus or minus 2 mm per second through the column.

• Mount the specimen vertically with its top edge at least 100 mm below the rim of the chimney.
• Set an initial oxygen concentration based on experience, for example 25 to 30 percent for flame-retardant EPR, and purge the column for at least 30 seconds.
• Apply the igniter flame to the top of the specimen in 5-second applications until sustained ignition is achieved, using top-surface ignition followed by the propagation mode where the standard requires it.
• Record whether the specimen burns for more than 3 minutes or whether the flame front passes the 50 mm reference mark (a reaction), or whether it self-extinguishes beforehand (no reaction).
• Vary the oxygen concentration in a bracketing sequence with steps of 0.5 to 2.0 percent until the ignition boundary is enclosed, typically consuming 5 to 15 specimens.

Calculation and Expression of Results

The LOI is calculated from the final bracketing oxygen concentrations. Under ISO 4589-2 the result is computed as LOI equals cF plus k times d, where cF is the final oxygen concentration in the bracketing series, d is the concentration step size, and k is a tabulated factor determined by the recorded response sequence; ASTM D2863 applies an analogous D-series and C-series procedure. The oxygen index is reported as volume percent oxygen to one decimal place, together with specimen dimensions, conditioning, ignition mode, and the standard used. Within-laboratory repeatability for a well-controlled EPR compound is typically better than 0.5 volume percent oxygen.

Factors Influencing LOI Results

• Specimen thickness: the oxygen index of EPR generally increases with thickness, so results from different dimensions are not directly comparable.
• State of cure: under-cured peroxide-crosslinked EPR gives inconsistent results; specimens must be fully vulcanized.
• Filler loading and dispersion: LOI rises steeply as ATH content increases above roughly 100 phr, and finer particle grades improve both LOI and mechanical balance; poor dispersion reduces flame-retardant efficiency.
• Test temperature: the oxygen index decreases significantly at elevated temperature, which is relevant for cables installed near heat sources and is addressed by ISO 4589-3.
• Aging effects: migration of plasticizers and antioxidants during thermal aging can lower the measured LOI.
• Equipment condition: gas purity, flowmeter calibration, and chimney cleanliness all affect measurement accuracy.

Applications

Oxygen index analysis directly supports the formulation, qualification, and quality control of EPR cable compounds used across power, industrial, transportation, and energy infrastructure markets. Typical end uses include:

• Low- and medium-voltage power distribution cables with EPR insulation, where flame-retardant inner sheaths and fills improve overall fire performance.
• Shipboard and offshore cables per the IEC 60092 series, requiring flame-retardant and low-smoke EPR sheathing.
• Nuclear power plant safety-class (1E) cables qualified to IEEE 383 and loss-of-coolant-accident profiles, where EPR is valued for radiation and thermal endurance.
• Railway rolling-stock and trackside cables, wind turbine cables, and renewable-energy systems in which flame retardancy is mandatory.
• Mining trailing cables, welding cables, portable power cords, and motor lead wires that combine EPR flexibility with flame-retardant jackets.

In production, LOI is routinely used for incoming compound acceptance, batch-to-batch formula audit, and failure analysis whenever flame performance deviates from specification.

FAQ

What oxygen index value qualifies an EPR cable compound as flame retardant?

Practical flame-retardant EPR grades are generally specified with an LOI of at least 28 percent, and enhanced halogen-free low-smoke sheathing compounds typically achieve 30 to 35 percent. Ordinary EPR insulation without mineral flame retardants measures only about 21 to 24 percent and cannot be classified as flame retardant.

Why does aluminum trihydrate raise the oxygen index of EPR?

ATH decomposes endothermically beginning near 200 to 230 degrees Celsius, absorbing approximately 1.2 kJ per gram and releasing roughly 35 percent of its mass as water vapor. The combined cooling, combustible-gas dilution, and residual oxide barrier effects suppress polymer pyrolysis, so a higher oxygen concentration is required to sustain combustion.

How does the oxygen index relate to cable flame tests such as IEC 60332?

LOI is a material-level screening parameter measured on molded strips, whereas IEC 60332-1, IEC 60332-2, and GB/T 18380 evaluate flame propagation on finished cable lengths. A high LOI of 28 to 30 percent strongly supports passing single-burner tests, but compliance must always be verified on the complete cable construction, including fillers, tapes, and armour.

Can the oxygen index be measured directly on finished cable samples?

Yes. Strips cut longitudinally from the insulation or sheath can be tested, provided the actual wall thickness is recorded, curvature is carefully flattened without stressing the rubber, and the edges are free of cuts or voids. The result applies only to the actual thickness tested.

Does crosslinking affect the oxygen index of EPR?

Crosslink density itself has only a secondary influence, often producing a slight increase in LOI. The dominant variables are the type and amount of flame-retardant filler, polymer ethylene content, and any combustible plasticizers present; nevertheless, specimens must always be fully cured to obtain reproducible values.